Generally, automatic gain control (AGC) has been sequentially performed only in a training symbol interval in an OFDM system.
A preamble of the IEEE 802.11a in the OFDM system includes short training symbols (Short Training Sequence) and long training symbols (Long Training Sequence). The short training symbol is utilized for signal recognition, automatic gain control, and coarse frequency offset estimation, and the long training symbol is utilized for fine symbol synchronization acquisition and fine frequency offset estimation.
An energy value of a data symbol is not constant because the data symbol is a result of Inverse Fast Fourier Transform (IFFT) for arbitrary data, compared with a training symbol in which electric power is normalized. For this reason, AGC needs to be performed by using training symbols in the preamble interval.
An AGC device of a general OFDM system measures energy of data, calculates its average value, transforms the average value into a dB value to be compensated, and performs feedback on a difference between the average value and the reference value to thus perform a gain control in the training symbol.
Reference electric power is an important parameter in gain control. Bit error rates (BER) of the OFDM system can be reduced, according to the value of the reference electric power, while gain control is performed. That is, the performance of the OFDM system depends on the reference electric power.
In the OFDM system, after the initial synchronization of the mobile station modem, AGC can be performed by frames using a preamble of a predetermined interval because a preamble is provided right after a synchronization signal.
However, neither a start point of the preamble nor a presence of the signal can be found before an execution of the initial synchronization. Also, Evaluation of the entire energy of a frame causes too much load for the hardware, and therefore there is a disadvantage in that two engines for before-initial synchronization and after-initial synchronization need to be provided independently.
As a prior art, Korean Patent Application No. 10-2001-29456, filed on May 28, 2001 discloses the invention entitled “Automatic gain control device of orthogonal frequency division multiplexing (OFDM) signal and an automatic gain control method using the device”.
In the OFDM transmission method using a repeated preamble for high-speed packet transmission, the above mentioned invention discloses an AGC device which detects a signal by monitoring signal power, and controls a signal gain rapidly and stably in the shortest time at the moment in which the signal is detected, in order to maintain an opt imal signal level for an analog/digital converter (ADC) input terminal.
It is not difficult to synchronize, without exact AGC, when there is no signal distortion in a good channel condition, but AGC must be performed to some degree when signal intensity is very weak due to a bad channel condition.
However, the prior invention has the disadvantage in that the device may not operate in a continuously bad channel condition since the device performs AGC after the signal is detected and only passively detects the signal in the same condition.
As to another prior art, U.S. Pat. No. 6,574,292 (filed on Jan. 17, 2002) discloses an invention entitled “Automatic gain control methods and apparatus suitable for use in OFDM receivers”.
More particularly, the above-noted invention discloses two stages of AGC method, which has a first stage of performing AGC by using a sample in the time domain and a second stage of performing AGC in the frequency domain.
However, the invention needs additional hardware for determining which stage of AGC method is to be performed, and has a disadvantage in that a gain value does not converge into an accurate value and errors occur since the invention performs AGC with two threshold values, which are a maximum threshold value and a minimum threshold value.
Hereafter, a conventional AGC method of the OFDM system will be described referring to FIG. 1 and FIG. 2.
FIG. 1 is a flowchart showing a conventional operation process of the AGC method by collecting samples in the time domain.
FIG. 2 is a flowchart showing a conventional operation process of the AGC method by collecting samples in the frequency domain.
The conventional AGC method of the OFDM system performs AGC in the time domain using a sample, and then performs AGC in the frequency domain using a sample.
Referring to FIG. 1, the AGC method includes measuring a signal level X of a sample in the time domain in step S101, determining whether the measured signal level X is greater than a maximum threshold value 1 in step S102, and decreasing the gain as much as A dB in step S103 when the measured signal level X is found to be greater than the maximum threshold value 1, and then the process returns to the previous step S101. Herein, the A dB is an arbitrary value, so it can be given appropriately.
The method includes determining whether the measured signal X is less than a minimum threshold value 1 in step S104 when the measured signal level X is not greater than the maximum threshold value 1, and increasing the gain as much as A dB in step S105 when the measured signal level X is found to be less than the minimum threshold value 1, and then the process returns to the previous step S101.
The method includes fixing the gain to a predetermined value in step S106 when the measured signal level X is not greater than the maximum threshold value 1, and also, is not less than the minimum threshold value 1, and then returning to step S101.
Referring to FIG. 2, the conventional AGC method in the frequency domain includes transforming the sample of the time domain into a sample of the frequency domain in step S201, and selecting appropriate tones in step S202.
The above mentioned AGC method includes measuring a signal level Y of the sample in the frequency domain in step S203, and measuring an error e in step S204. The error e is generated by subtracting a reference value from the signal level Y.
The method includes evaluating an average value of the error e in step S205, and determining whether the signal level Y is greater than a maximum threshold value 2 in step S206. The gain is decreased as much as A dB in step S207, when the signal level Y is greater than the maximum threshold value 2, and the process returns to step S201.
The method includes determining whether the signal level Y is less than a minimum threshold value 2 in step S208 when the measured signal level Y is not greater than the maximum threshold value 2. The gain is increased as much as A dB in step S209 when the case that the signal level Y is less than the minimum threshold value 2, and the process returns to step S201.
The gain is fixed in step S210 when the signal level Y is not greater than the maximum threshold value 2, and also, is not less than the minimum threshold value 2, and then the process returns to step S201.
However, the method shown in FIG. 1 and FIG. 2 needs additional hardware for determining which stage will be used for AGC between the first stage of the time domain and the second stage of the frequency domain. Furthermore, the method has the disadvantage that the gain value does not converge into an accurate value and errors occur because the invention performs AGC with two threshold values, which are a maximum threshold value and a minimum threshold value.
The information disclosed in this Background Art interval is only for enhancement of understanding of the background art, and therefore, unless explicitly described to the contrary, it should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.